Hitherto, to zinc-plated steel sheets, aluminum-plated steel sheets and the like, a rust prevention treatment by chromate, which uses hexavalent chromate, has been widely applied, and, as required, theses steel sheets have been coated with an organic resin or in addition to this, they have been top coated with various coating compositions in order to provide a higher degree of corrosion resistance, an anti-fingerprint property, an anti-scratch property and lubricity.
In recent years, there are moves to regulate or prohibit chromate treatments which hitherto have been applied to steel materials by a law against the background of increasing environmental issues. Since a chromate treated layer in itself has a high degree of corrosion resistance and an adhesion property to a paint, if this chromate treatment is not performed, it is expected that these properties are significantly deteriorated. Accordingly, it has been desired to form a rust preventing layer having good corrosion resistance and a good adhesion property to a paint by only a one-step treatment by an organic resin without carrying out a surface preparation by a chromate treatment.
In Japanese Kokai Publication 2001-164182, a rust prevention coating agent containing a water-borne resin composition obtained by reacting a composition containing a water-borne polyurethane resin, a water-borne polyolefin resin, water-dispersible silica and a silane coupling agent and/or a condensate of hydrolysate thereof, a thiocarbonyl group-containing compound and a phosphate ion, a rust prevention method of coating this rust prevention coating agent, and a metal material, to which rust prevention is applied, coated with the rust prevention coating agent are disclosed. However, in accordance with the above method, there are problems that since the reaction of the silane coupling agent with the respective resin is insufficient, a degree of compounding among the respective components in a coat to be formed is low and therefore solvent resistance and alkali resistance are low.
In Japanese Kokai Publication 2001-199003, a coated steel sheet having a composite coat containing two species of resins having different solubility parameters and inorganic components is described. However, because an inadequately crosslinked coat is formed, there is a problem of being suffered from large damage in a solvent rubbing test.
In Japanese Kokai Publication 2001-214283, a surface treated zinc-based plated steel sheet, which has a coat layer formed by applying a water-borne composition containing a metal compound, a water-soluble organic resin and acid onto the surface of a zinc-based plated steel sheet, is disclosed. However, because an amount of a carboxyl group in the water-soluble resin is relatively large, there is a problem that alkali resistance is poor.
In Japanese Kokai Publication 2003-49281, a coated steel sheet having an organic coat formed from an epoxy resin and a glycoluril resin on at least one side of a metal sheet is disclosed. However, there is a problem that because the formed organic coat does not contain a rust preventive agent, corrosion resistance is inadequate.
In Japanese Kokai Publication 2003-105555, a surface treated steel sheet, which has a surface treatment coat formed from a surface treatment composition containing (a) water-dispersible resin and/or a water-soluble resin, (b) a silane coupling agent, and (c) phosphoric acid and/or hexafluorometalic acid on the surface of a zinc-based plated steel sheet or an aluminum-based plated steel sheet, is disclosed. However, there is a problem that the stability of a treatment agent or the solvent resistance of a coat obtained is inadequate.
In Japanese Kokai Publication 2005-281863, a coated steel sheet, which is provided with a coating containing a crosslinked resin matrix and an inorganic rust preventive agent, is described. But, since in this coated steel sheet, all of performance such as corrosion resistance of a processed portion, tape peeling resistance, solvent resistance, alkali resistance and abrasion resistance are not necessarily satisfactory, a coated steel sheet in which these performance are improved is required.
In Japanese Kokai Publication 2001-288582, a surface treated metal sheet treated with a water-borne organic composite coating composition containing a water-borne resin, colloidal silica and a lubricant is described. However, since this invention is substantially predicated on using a chromate treatment as surface preparation, if only a coat composed of the organic composite coating composition is employed, only insufficient performance can be attained in the corrosion resistance or adhesion though satisfactory performance can be attained in the abrasion resistance. Accordingly, a coated steel sheet, performance of which is further improved, is required.